The long-term objective of the proposed research is to understand in detail the phototransduction mechanism in retinal cones. The specific aims are: 1) to measure the rate constant of spontaneous isomerization of human red cone pigment with 11-cis retinal as chromophore in a transgenic mouse line, in order to assess its importance in human cones. 2) to measure and analyze the response properties and background-adaptation behavior of goldfish red, green and blue cones and to correlate these parameters with the separately measured dark phosphodiesterase activity in each cone type (from which the spontaneous isomerization rate of each pigment type can also be obtained). Goldfish retina has the advantage that it has all three cone types like human retina, and the correlate of biochemical studies by others. 3) to examine the 50-year-old question of whether there is a strict quantitative relation for various pigments between the lambda-max of the action spectrum and the activation energy of photoisomerization, and to correlate this information with the energetics of the associated spontaneous isomerization activity in order to firmly establish photoisomerization and thermal isomerization as fundamentally the same process. 4) to examine the contributions of the pigment kinase (GRK1 and 7), arrestin, and the GAP protein RGS9 to the response properties of cones. We shall employ morpholinos to knockdown each transduction protein in turn in Xenopus and see how this affects the cone response to light. The proposed work will lead to a much greater understanding of the light response of cones, which are more important than rods for human vision. The gained knowledge will also help understand and treat human diseases affecting cone function.